1. Field of the Invention
The present invention relates to network intermediate devices for interconnecting shared and point-to-point networks, and specifically to a configurable WAN/LAN bridge.
2. Description of Related Art
Communications networks consist of terminals that are interconnected by links to network intermediate devices such as repeaters, switches, bridges and the like. The intermediate devices are used to interconnect the terminals to establish paths of communication among the terminals.
As network technologies have developed, a wide variety of techniques have evolved for operating links between terminals and intermediate devices. For example, the Ethernet network standard according to the IEEE Standard 802.3, published by the Institute of Electrical and Electronic Engineers, supports a variety of local area network (LAN) technologies. These technologies include the 10 BASE-T standard which specifies a 10 megabit per second carrier sense multiple access with collision detection (CSMA/CD) LAN over two pairs of twisted pair telephone wire, the 10 BASE-F standard which specifies a 10 megabit per second CSMA/CD LAN over fiber optic cable; the 100 BASE-TX standard specifies a 100 megabit per second CSMA/CD LAN over two pairs of category five UTP or shielded twisted pair wire, and a number of other current and emerging technologies. Typically data terminals in a network are designed to support a specific LAN technology. However, network intermediate devices which are designed to interconnect a wide variety of terminals, need a technique for resolving the particular technology used across a link coupled to a particular port of the device.
A CSMA/CD protocol has been widely adopted in a 10 megabit per second Mbs local area network standard known as Ethernet (ANSI/IEEE Standard 802.3). The 10 Mbps standard has been extended to 100 Mbs embodiments, as set forth in IEEE Standard 802.3u. Furthermore, the investigations are underway to extend the data rate of the CSMA/CD protocol even higher, for example up to 1 gigabit per second.
According to the CSMA/CD protocol, an end station has a medium access control (MAC) unit which, in order to transmit a packet first senses the status of the transmission medium to which it is attached to determine whether a carrier is present or not. If no carrier is present, then the MAC unit begins transmission of the packet. Because of the delay from transmission of a packet until a packet is received by a remote end station in the network, the possibility of collisions occurs. According to the CSMA/CD protocol, the MAC unit at each end station is capable of detecting such collisions. Thus, after beginning to transmit a packet, the MAC unit will continue to monitor the transmission medium to detect collisions. In order to ensure that all collisions are detected, the MAC unit must monitor for collisions during the transmission of the entire packet. Furthermore, if the packet is relatively short, there is a possibility that collisions may occur even after the MAC unit is finished sending the short packet. Thus, the MAC unit must continue to monitor for collisions for a finite amount of time after it begins transmission, often referred to as a slot time.
Thus, it can be seen that the consequences of collisions in the CSMA/CD based network can result in a significant amount of data being transmitted in the network at the expense of efficient use of the network. Prior art systems have developed to improve the efficiency of CSMA/CD networks. However, the prior art approaches come at the expense of more complicated network intermediate devices.
There is considerable software and hardware overhead associated with connecting networks. Physical and packet protocols must be matched. When the networks being connected are LAN and WAN, the former with a shared communication medium and the latter with a point-to-point communication medium additional overhead is present. The packets on the WAN link require additional processing to extract source and header information in order to place them on a LAN. This type of intra-network connection is particularly prevalent today as many corporations connect their LANs to the internet. To meet the challenge of this environment better cross network devices must be developed. Existing bridge and router technology presents a severe bottleneck to corporate communications with the outside world.
Improvements are needed in the way in which network connection devices handle the delays inherent in the network scheme required for management and resolution of collisions. Furthermore, it is desirable for such system to operate without increasing the minimum frame size applied to the network, and without propagating inflated collision fragments over the high speed to lower speed interfaces in the network. In addition, it is desirable to provide such improved system without requiring complex changes to existing MAC units.